The Syrian Observatory for Human Rights said the attack occurred on Wednesday evening as the militiamen were en route to search for a shepherd who had been kidnapped and subsequently killed by the IS jihadists.
The Observatory, a Britain-based monitor with sources in Syria, reported a death toll of eight, including "six members of the National Defence Forces and two sheep herders".
IS overran large swathes of Syria and Iraq in 2014, proclaiming a so-called caliphate and launching a reign of terror.
It was defeated territorially in Syria in 2019, but its remnants still carry out deadly attacks -- particularly in the Badia desert -- and mainly targeting government loyalists and Kurdish-led fighters.
The desert runs from the outskirts of Damascus to the Iraqi border.
Last month, the Observatory said IS fighters had killed nearly 4,100 people in Syria since its so-called caliphate fell in 2019 -- more than half of them in the Badia.
The United Nations in January said IS's combined strength in Iraq and Syria was 3,000-5,000 fighters, with the Badia serving as a hub for the group in Syria.
Syria's war has killed more than half a million people and displaced millions more since it erupted in March 2011 with Damascus's brutal repression of anti-government protests.